A variety of nanostructured surfaces, including nanopillars, nanocones, nanowires, inverted pyramids, nanospheres, nanoshells, and nanodomes, have enabled strong, broadband absorption in semiconductors for optoelectronic devices such as photosensors and photovoltaics. Dielectric nanostructures are extremely effective at coupling and directing light across an interface between the outside world and optically active materials owing to a rich set of optical phenomena, including gradual refractive index matching, scattering, and coupling to guided modes. However, in many applications, a nanostructured optical interface must also serve as a conduit for electrical current between the semiconductor and an external circuit. In addition to providing photon management, the interface must offer high electrical conductivity.
Transparent conductive oxides offer the industry standard performance for conductive layers with high optical transparency, but are limited because of low conductivity in the film, slow deposition processes, and ultimately because of material scarcity of some elements in the oxide. A number of alternatives based on metal nanowire networks have emerged, but these are limited to low surface coverage to ensure high coupling through the photon management interface. Extraordinary transmission, i.e., transmission in excess of that predicted by diffraction theory, has been observed in high-surface coverage metal films with subwavelength holes. However, the transmission through these surfaces is generally limited to a narrow range of wavelengths and polarizations of light.